Flattened optical frequency-locked multi-carrier generation by one dml and one phase modulator

ABSTRACT

The disclosure relates to a method for optical frequency-locked multi-carrier generation based on one directly-modulated laser (DML) and one phase modulator (PM) in cascade driven by sinusoidal waveform (at the same or different frequency). When the DML and PM is driven by the same frequency RF signal at 12.5 GHz, adopting this method, 16 optical subcarriers with 12.5-GHz frequency spacing are generated with power difference less than 3 dB. When the DML and PM is driven by the different frequency with DML at 12.5 Ghz and PM at 25 GHz, over 24 optical subcarriers are generated with 12.5-GHz frequency spacing and amplitude fluctuation less than 3 dB. The number of the generated optical subcarriers can be further increased when the driving power for the DML is increased.

BACKGROUND OF THE INVENTION

The optical coherent and frequency-locked multi-carrier generation isone of the key techniques for the realization of superchannel, which isa promising candidate for future high-speed optical systems andnetworks. The flattened comb generation based on cascaded intensitymodulator (IM) and phase modulator (PM) has been intensively studied anddemonstrated. Recently, we have demonstrated flattened comb generationusing only phase modulators driven by fundamental frequency sinusoidalsources with small frequency offset. Also, a lot of schemes forflattened comb generation have been demonstrated based onin-phase/quadrate (I/Q) modulator combined with recirculating frequencyshifter (RFS). On the other hand, it is well known that relative to IMor PM, directly-modulated laser (DML) has the advantages of compactsize, low power consumption and easy integration. In this paper, wepropose and experimentally demonstrate a novel scheme for opticalfrequency-locked multi-carrier generation based on one DML and one PM incascade driven by sinusoidal waveform at the same frequency or differentfrequency. When DML and PM are driven by at the same frequency, adoptingthis scheme, 16 optical subcarriers with 12.5-GHz frequency spacing aresuccessfully generated with power difference less than 3 dB. When theDML and PM is driven by the different frequency with DML at 12.5 Ghz andPM at 25 GHz, we experimentally demonstrate that over 24 opticalsubcarriers can be generated with 12.5-GHz frequency spacing andamplitude fluctuation less than 3 dB. Furthermore, the number of thegenerated optical subcarriers can be further increased when we increasethe driving power for the DML.

BRIEF SUMMARY OF THE INVENTION

We propose and experimentally demonstrate a novel scheme for opticalfrequency-locked multi-carrier generation based on onedirectly-modulated laser (DML) and one phase modulator (PM) in cascadedriven by sinusoidal waveform (at the same frequency or different). Whenthe DML and PM is driven by the same frequency RF signal at 12.5 GHz,adopting this scheme, 16 optical subcarriers with 12.5-GHz frequencyspacing are successfully generated with power difference less than 3 dB.When the DML and PM is driven by the different frequency with DML at12.5 Ghz and PM at 25 GHz, we experimentally demonstrate that over 24optical subcarriers can be generated with 12.5-GHz frequency spacing andamplitude fluctuation less than 3 dB. Furthermore, the number of thegenerated optical subcarriers can be further increased when we increasethe driving power for the DML.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference is nowmade to the accompanying drawings, which are not necessarily drawn toscale. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate disclosedembodiments and/or aspects and, together with the description, serve toexplain the principles of the invention, the scope of which isdetermined by the claims.

FIG. 1 is a schematic diagram of comb generation based on (a) PM-onlyand (b) cascaded DML and PM.

FIG. 2 is an experimental setup for optical multi-carrier generation.

FIG. 3 shows optical spectra (0.01-nm resolution) (a) without and (b)with driving signal.

FIG. 4 shows optical spectra (0.01-nm resolution). (a) After PM; (b)after TOF.

FIG. 5 shows schematic structure and output spectrum of comb generationbased on (a) PM-only driven by low-frequency RF clock, (b) cascaded DMLand PM driven by the same RF clock, (c) PM-only driven by high-frequencyRF clock and (d) cascaded DML and PM driven by different RF clock.

FIG. 6 shows an experimental setup for optical multi-carrier generationbased on cascaded DML and PM driven by different RF clock.

FIG. 7 shows optical spectra (0.01-nm resolution). (a) DML withoutdriving signal; (b) DML with 26-dBm driving signal; (c) DML withoutdriving signal and PM with 30-dBm driving signal; (d) DML with 26-dBmdriving signal and PM with 30-dBm driving signal; (e) after PM-TOF whenDML with 26-dBm driving signal and PM with 30-dBm driving signal.

FIG. 8 shows optical spectra (0.01-nm resolution). (a) DML with 29-dBmdriving signal; (b) DML with 29-dBm driving signal and PM with 30-dBmdriving signal; (c)-(d) after PM-TOF when DML with 29-dBm driving signaland PM with 30-dBm driving signal.

DETAILED DESCRIPTION OF THE INVENTION Case 1: DML and IM are Driven atthe Same Frequency

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some examples of theembodiments of the inventions are shown. It is to be understood that thefigures and descriptions provided herein may have been simplified toillustrate elements that are relevant for a clear understanding of thepresent invention, while eliminating, for the purpose of clarity, otherelements found in typical optical coherent and frequency-lockedmulti-carrier generation system and methods. Those of ordinary skill inthe art may recognize that other elements and/or steps may be desirableand/or necessary to implement the devices, systems, and methodsdescribed herein. However, because such elements and steps are wellknown in the art, and because they do not facilitate a betterunderstanding of the present invention, a discussion of such elementsand steps may not be provided herein. The present disclosure is deemedto inherently include all such elements, variations, and modificationsto the disclosed elements and methods that would be known to those ofordinary skill in the pertinent art. Indeed, these disclosure inventionsmay be embodied in many different forms and should not be construed aslimited to the embodiments set forth therein; rather, these embodimentsare provided by way of example so that this disclosure will satisfyapplicable legal requirements. Like numbers refer to like elementsthroughout.

It is well known that sinusoidal phase modulation of a narrow-band CWlaser can create a frequency comb with high repetition rate, tunablefrequency spacing and stable optical central frequency. As shown in FIG.1( a), when one PM driven by a sinusoidal waveform at f_(s) is used tomodulate the CW lightwave at f_(c), the output electrical field of thePM can be expressed as

$\begin{matrix}\begin{matrix}{{E_{out}(t)} = {K\mspace{14mu} {\exp \left( {{j2\pi}\; f_{c}t} \right)}{\exp \left\lbrack {{j\kappa sin}\left( {2\pi \; f_{s}t} \right)} \right\rbrack}}} \\{= {K{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}(\kappa)}{\exp \left\lbrack {{{j2\pi}\left( {f_{c} + {nf}_{s}} \right)}t} \right\rbrack}}}}}\end{matrix} & (1)\end{matrix}$

Where J_(n) is the first kind Bessel function of order n, k is themodulation index of the PM, and K is a constant irrelevant to ourdiscussion. The disadvantage for comb generation based on only PM isquite poor spectral flatness. Furthermore, the limited driving voltageof PM and the limitation of electrical amplifiers (EAs) significantlylimit the modulation index of PM and the number of the generated opticalsubcarriers.

In order to overcome the disadvantages of the PM-only scheme, we proposethe novel cascaded PM and DML scheme, just as shown in FIG. 1( b). Whenbiased at a large direct current (DC) and driven by a sinusoidalwaveform at f_(s), the output electrical field of a DML at f_(c) can beexpressed as

E _(out1)(t)≈K[1+κ₁ sin(2πf _(s) t)]exp(j2πf _(c) t)  (2)

Where k₁ is the modulation index of the DML. Here, the inherent chirpfrom the DML is largely removed and can be neglected due to the adoptionof the large DC bias. Thus, the output electrical field of the PM drivenby the synchronous sinusoidal waveform at f_(s) can be expressed as

$\begin{matrix}{{{E_{{out}\; 2}(t)} \approx {{K\left\lbrack {1 + {\kappa_{1}{\sin \left( {2\pi \; f_{s}t} \right)}}} \right\rbrack}{\exp \left( {{j2\pi}\; f_{c}t} \right)}{\exp \left\lbrack {{j\kappa}_{2}{\sin \left( {2\pi \; f_{s}t} \right)}} \right\rbrack}}} = {{K{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}{\exp \left\lbrack {{{j2\pi}\left( {f_{c} + {nf}_{s}} \right)}t} \right\rbrack}}}} - {j\; K\frac{\kappa_{1}}{2}{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}\exp \left\{ {{{j2\pi}\left\lbrack {f_{c} + {\left( {n + 1} \right)f_{s}}} \right\rbrack}t} \right\}}}} + {j\; K\frac{\kappa_{1}}{2}{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}\exp \left\{ {{{j2\pi}\left\lbrack {f_{c} + {\left( {n - 1} \right)f_{s}}} \right\rbrack}t} \right\}}}}}} & (3)\end{matrix}$

Where k₂ is the modulation index of the PM. Compared to Eq. (1), theright second and third terms of Eq. (3) can flatten the amplitude of thegenerated optical subcarriers spaced at f_(s). Furthermore, theadvantages of DML, such as low cost, compact size, low power consumptionand so on, make the cost and integration of our proposed scheme muchmore efficient.

FIG. 2 shows the experimental setup for the flattened opticalmulti-carrier generation based on our proposed cascaded DML and PMscheme. For optical multi- carrier generation, a 12.5-GHz sinusoidalradio-frequency (RF) signal is first equally halved into two branches bya power divider. Next, one branch is power amplified to 24 dBm and usedto drive the DML, while the other is power amplified to 30 dBm and usedto drive the PM. The phase shifter (PS) before the PM is used tosynchronize the two branches. The DML is a commercially availabledistributed-feedback (DFB) laser and has 74-mA DC bias and 9.3-dBmaverage output power. The polarization-maintaining Erbium-doped fiberamplifier (EDFA) between the cascaded DML and PM is used to compensatefor the modulation loss. The subsequent polarization-maintaining tunableoptical filter (TOF) with tunable bandwidth and wavelength is used tosuppress the amplified spontaneous emission (ASE) noise from thepolarization-maintaining EDFA. FIG. 3( a) and (b) show the opticalspectra (0.01-nm resolution) after the DML without and with drivingsignal, respectively. FIG. 4( a) and (b) show the optical spectra(0.01-nm resolution) after PM and TOF, respectively. 16 opticalsubcarriers with 12.5-GHz frequency spacing are generated with amplitudefluctuation less than 3 dB.

The cascaded TOF and 12.5/25-GHz optical inter-leaver (IL) is used tochoose the desired optical subcarrier from the generated 16 opticalsubcarriers.

Case II: DML and IM are Driven at the Difference Frequency

As shown in FIG. 5( a), when one PM driven by a sinusoidal RF clock atf_(s) is used to modulate the CW lightwave at f_(c), the outputelectrical field of the PM can be expressed as

$\begin{matrix}\begin{matrix}{{E_{out}(t)} = {K\mspace{14mu} {\exp \left( {{j2\pi}\; f_{c}t} \right)}{\exp \left\lbrack {{j\kappa sin}\left( {2\pi \; f_{s}t} \right)} \right\rbrack}}} \\{= {K{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}(\kappa)}{\exp \left\lbrack {{{j2\pi}\left( {f_{c} + {nf}_{s}} \right)}t} \right\rbrack}}}}}\end{matrix} & (4)\end{matrix}$

Where J_(n) is the first kind Bessel function of order n, k is themodulation index of the PM, and K is a constant irrelevant to ourdiscussion. The subcarrier spacing is f_(s). The disadvantage for combgeneration based on only PM is quite poor spectral flatness.Furthermore, the limited driving voltage of PM and the limitation ofelectrical amplifiers (EAs) significantly limit the modulation index ofPM and the number of the generated optical subcarriers.

In order to overcome the disadvantages of the PM-only scheme, we proposethe novel cascaded PM and DML scheme just as shown in FIG. 5( b). Whenbiased at a large DC and driven by a sinusoidal RF clock at f_(s), theoutput electrical field of a DML a f_(c) can be expressed as

E _(out1)(t)≈K[1+κ₁ sin(2πf _(s) t)]exp(j2πf _(c) t)  (5)

Where k₁ is the modulation index of the DML. Here, the inherent chirpfrom the DML is largely removed and can be neglected due to the adoptionof the large DC bias. Thus, the output electrical field of thesubsequent PM driven by the synchronous sinusoidal RF clock at f_(s) canbe expressed as

$\begin{matrix}{{{E_{{out}\; 2}(t)} \approx {{K\left\lbrack {1 + {\kappa_{1}{\sin \left( {2\pi \; f_{s}t} \right)}}} \right\rbrack}{\exp \left( {{j2\pi}\; f_{c}t} \right)}{\exp \left\lbrack {{j\kappa}_{2}{\sin \left( {2\pi \; f_{s}t} \right)}} \right\rbrack}}} = {{K{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}{\exp \left\lbrack {{{j2\pi}\left( {f_{c} + {nf}_{s}} \right)}t} \right\rbrack}}}} - {j\; K\frac{\kappa_{1}}{2}{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}\exp \left\{ {{{j2\pi}\left\lbrack {f_{c} + {\left( {n + 1} \right)f_{s}}} \right\rbrack}t} \right\}}}} + {j\; K\frac{\kappa_{1}}{2}{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}\exp \left\{ {{{j2\pi}\left\lbrack {f_{c} + {\left( {n - 1} \right)f_{s}}} \right\rbrack}t} \right\}}}}}} & (6)\end{matrix}$

Where k₂ is the modulation index of the PM. Compared to Eq. (4), theright second and third terms of Eq. (6) can flatten the amplitude of thegenerated optical subcarriers spaced at f_(s). As a result, we canrealize flattened optical multi-carrier generation based on the cascadedDML and PM driven by the same RF clock shown in FIG. 5( b).

Also, it is well known that the modulation bandwidth of the DML isrelatively narrow, such as about 10 GHz, while that of the PM can beover 40 GHz. Thus, compared to the DML, the PM can be driven by ahigher-frequency RF clock. As shown in FIG. 5( c), when one PM driven bya sinusoidal RF clock at 2f_(s) is used to modulate the CW lightwave atf_(c), the output electrical field of the PM can be expressed as

$\begin{matrix}\begin{matrix}{{E_{out}^{\prime}(t)} = {K\mspace{14mu} {\exp \left( {{j2\pi}\; f_{c}t} \right)}{\exp \left\lbrack {{j\kappa sin}\left( {4\pi \; f_{s}t} \right)} \right\rbrack}}} \\{= {K{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}(\kappa)}{\exp \left\lbrack {{{j2\pi}\left( {f_{c} + {2{nf}_{s}}} \right)}t} \right\rbrack}}}}}\end{matrix} & (7)\end{matrix}$

Similarly, the generated optical subcarriers spaced at 2f_(s) also havequite poor spectral flatness. When we further introduce the DML drivenby a RF clock at f_(s), just as shown in FIG. 5( d), the final outputelectrical field can be expressed as

$\begin{matrix}{{{E_{{out}\; 2}^{\prime}(t)} \approx {{K\left\lbrack {1 + {\kappa_{1}{\sin \left( {2\pi \; f_{s}t} \right)}}} \right\rbrack}{\exp \left( {{j2\pi}\; f_{c}t} \right)}{\exp \left\lbrack {{j\kappa}_{2}{\sin \left( {4\pi \; f_{s}t} \right)}} \right\rbrack}}} = {{K{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}{\exp \left\lbrack {{{j2\pi}\left( {f_{c} + {2{nf}_{s}}} \right)}t} \right\rbrack}}}} - {j\; K\frac{\kappa_{1}}{2}{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}\exp \left\{ {{{j2\pi}\left\lbrack {f_{c} + {\left( {{2n} + 1} \right)f_{s}}} \right\rbrack}t} \right\}}}} + {j\; K\frac{\kappa_{1}}{2}{\sum\limits_{n = {- \infty}}^{\infty}{{J_{n}\left( \kappa_{2} \right)}\exp \left\{ {{{j2\pi}\left\lbrack {f_{c} + {\left( {{2n} - 1} \right)f_{s}}} \right\rbrack}t} \right\}}}}}} & (8)\end{matrix}$

The subcarrier spacing is f_(s). Compared to Eq. (7), the right secondand third terms of Eq. (8) can not only flatten the amplitude of thegenerated optical subcarriers, but also introduce new spectralcomponents and thus double the number of the generated opticalsubcarriers. That is, if the number of the generated flattened opticalsubcarriers is theoretically n for the scheme that both the DML and thePM are driven at f_(s) as shown in FIG. 5( b), the number of thegenerated flattened optical subcarriers will be 2n for the scheme thatthe DML is driven at f_(s) and the PM at 2f_(s) as shown in FIG. 5( d).Moreover, the advantages of DML, such as low cost, compact size, lowpower consumption and so on, make the cost and integration of ourproposed cascaded DML and PM scheme much more efficient.

FIG. 6 shows the experimental setup for the flattened opticalmulti-carrier generation based on the cascaded DML and PM driven bydifferent RF clock. A 12.5-GHz sinusoidal RF clock is equally halvedinto two branches by a power divider. One branch first passes through anEA and is power amplified to 24˜29 dBm to drive an DML, while the otherfirst passes through a phase shifter (PS), an active frequency doubler(×2) and an EA in serial, and is power amplified to 30 dBm to drive anPM. Here, the PS is used to synchronize the two branches. The DML is acommercially available distributed-feedback (DFB) laser and has athreshold current of 24 mA and modulation bandwidth of over 10 GHz aswell as 111-mA DC bias and 10.3-dBm average output power. The PM has 3-Vhalf-wave voltage, 3-dB insertion loss and 32-GHz modulation bandwidth.The polarization-maintaining Erbium-doped fiber amplifier (PM-EDFA)between the cascaded DML and PM is used to compensate for the modulationloss. The subsequent polarization-maintaining tunable optical filter(PM-TOF) with tunable bandwidth and wavelength is used to suppress theamplified spontaneous emission (ASE) noise from the PM-EDFA.

FIG. 7( a) shows the output optical spectrum (0.01-nm resolution) of theDML without driving signal. It can be seen that the side modesuppression ratio (SMSR) of the DML is over 40 dB. FIG. 7( b) shows theoutput optical spectrum (0.01-nm resolution) of the DML with 26-dBmdriving signal. The optical spectrum is asymmetrical at 1538.1-nmcentral wavelength due to the inherent chirp from the DML [13]. Themulti-peak effect at 1539.5 nm is caused by the residual side mode fromthe DML. The adoption of an optical Fiber Bragg Grating (FBG) filter canremove the side mode and thus improve the performance of the DML. FIG.7( c) and FIG. 7( d) show the output optical spectra (0.01-nmresolution) of the PM driven at 30 dBm, while for the former the DML hasno driving signal and for the latter the DML is driven at 26 dBm. It canbe seen that, the generated optical subcarriers based on only phasemodulation has quite poor spectral flatness, which can be greatlyimproved by the introduction of direct intensity modulation. Theintroduction of direct intensity modulation also halves the subcarrierspacing from 25 GHz to 12.5 GHz and thus doubles the number of thegenerated optical subcarriers, which agrees well with the previousformulaic analysis. It is worth noting that the lower optical power ofthe optical subcarriers around 1539.1 nm is related to the inherentchirp and the side mode from the DML. FIG. 7( e) shows the opticalspectrum (0.01-nm resolution) after the PM-TOF when the DML is driven at26 dBm and the PM is driven at 30 dBm. It can be seen that the cascadedDML and PM scheme can generate 20 optical subcarriers with amplitudefluctuation less than 3 dB, or 26 optical subcarriers with amplitudefluctuation less than 5 dB. The neighboring frequency spacing is 12.5GHz.

When the DML is driven at 29 dBm and the PM is driven at 30 dBm, FIG. 8(a) and FIG. 8( b) respectively show the output optical spectra (0.01-nmresolution) of the DML and the PM, while FIG. 7( c) and FIG. 7( d) afterfurther PM-TOF with different bandwidth. It can be seen that thecascaded DML and PM scheme can generate 24 optical subcarriers withamplitude fluctuation less than 3 dB. The neighboring frequency spacingis 12.5 GHz. Compared FIG. 8 with FIG. 7, we can conclude that thenumber of the generated optical subcarriers can be further increasedwhen we increase the driving power for the DML.

Although the invention has been described and illustrated in exemplaryforms with a certain degree of particularity, it is noted that thedescription and illustrations have been made by way of example only.Specific terms are used in this application in a generic and descriptivesense only and not for purposes of limitation. Numerous changes in thedetails of construction and combination and arrangement of parts andsteps may be made. Accordingly, such changes are intended to be includedin the invention, the scope of which is defined by the claims.

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1. A system for optical frequency-locked multi-carrier generationcomprising: one directly-modulated laser (DML), one phase modulator (PM)in cascade driven by a sinusoidal waveform, and one phase shifter (PS)to synchronize the driving signal of the DML and the PM.
 2. The systemof claim 1, wherein the DML and PM are driven by same frequency RFsignal.
 3. The system of claim 1, wherein the DML and the PM are drivenby different frequency RF signal, where the driving voltage on the PM isabout three times of half-wave voltage of the PM.
 4. The system of claim3, wherein a driving voltage on the DML is tunable to flat the amplitudeof subcarriers.
 5. The system of claim 4, wherein a large DC bias on theDML is used to reduce the transit chirp.
 6. The system of claim 2,wherein the DML and PM are driven by the same frequency RF signal, whereat least 16 optical subcarriers are generated.
 7. The system of claim 3,wherein the DML and PM are driven by the different frequency, where atleast 24 optical subcarriers are generated.
 8. The system of claim 1,wherein the DML driven by the sinusoidal waveform is characterized byEq. (2).
 9. The system of claim 1, wherein the PM driven by thesinusoidal waveform is characterized by Eq. (3)
 10. A method for opticalfrequency-locked multi-carrier generation comprising: driving onedirectly-modulated laser (DML) and one phase modulator (PM) in cascadeby a sinusoidal waveform, and synchronizing the driving signal of theDML and the PM by one phase shifter (PS).
 11. The method of claim 10,wherein the DML and PM are driven by same frequency RF signal.
 12. Themethod of claim 10, wherein the DML and PM are driven by differentfrequency RF signal, where the driving voltage on the PM is about threetimes of half-wave voltage of the PM.
 13. The method of claim 12,wherein a driving voltage on the DML is tunable to flat the amplitude ofsubcaniers.
 14. The method of claim 13, wherein a large DC bias on theDML is used to reduce the transit chirp.
 15. The method of claim 11,wherein the DML and PM are driven by the same frequency RF signal, whereat least 16 optical subcarriers are generated.
 16. The method of claim12, wherein the DML and PM are driven by the different frequency, whereat least 24 optical subcarriers are generated.
 17. The method of claim10, wherein the DML driven by the sinusoidal waveform is characterizedby Eq. (2).
 18. The method of claim 10, wherein the PM driven by thesinusoidal waveform is characterized by Eq. (3)